Underground solutions, The Bekaert Seminar on steel-fibre reinforcement, was held at the Institution of Civil Engineers in London on 29 October 2019. Kicking-off the proceedings was a presentation on the design of steel fibre-reinforced concrete (SFRC) structures given by Remy Roland, Bekaert South Europe technical manager.

Explaining that steel fibres transfer forces across cracks in concrete, Mr Roland highlighted their material characteristics, design values, section design and structural SFRC design. He said that in terms of parameters influencing mechanical performance of 3D, 4D and 5D with the same lengths and diameters, pullout tests for Dramix show that for 3D and 4D, ductility comes from a controlled sliding of the fibre within the concrete matrix. Structural design can be achieved with steel fibre only and a combination of steel fibres plus rebar and mesh.

Benoit De Rivaz, Bekaert Global Technical Manager

Illustrating both the advantages and challenges, Benoit De Rivaz gave a lively presentation on specifying and testing fibrereinforced sprayed concrete. Available in various shapes, sizes and materials, most fibre used in concrete falls into three main groups:

  • Steel fibres typically 30-70mm long and 0.5-1mm diameter with straight, wavy or hooked ends;
  • Micro synthetic fibres, usually 5-20mm long, 0.03mm diam or less with monofilament or fibrillated fibres, e.g polypropylene.
  • Macro synthetic fibres, these can be 30-70mm long and 0.5- 1mm in diameter. They may be embossed or wavy and are typically polypropylene.

De Rivaz cited standard NF EN 14487-1 (Annex A): ‘The residual strength must be specified when the concrete characteristics are used in a structural design model.

The energy absorption value measured on a plate with continuous support can be specified when, in the case of rock bolting, emphasis is laid on energy which has to be absorbed during the deformation on the rock.’

Regarding the ‘energy absorption’, the EN 14488-5 Panel Test or Norway round plate test on continuous support was proposed because this test simulates (at a laboratory scale) the structural behaviour of the system anchor bolt – sprayed concrete under flexural and shear load. A shotcrete tunnel lining behaves like a slab. The hyperstatic test conditions allow load redistribution. The test can also be carried out with mesh reinforcement.

Questioned as to whether the energy absorption value is synonymous with on-site safety, De Rivaz explained: “If the concrete matrix is very strong, the energy may be high, even though the force rapidly falls after the elastic phase”. Consequently, it is thought advisable to increase the level of energy absorption according to the concrete class and also criteria of 5mm.

De Rivaz also discussed how sprayed concrete lining (SCL) is evolving. The main design change drivers are permanent sprayed concrete; automated, mechanised construction; safety culture-limited man access; sprayed waterproof membranes; surveying technology fibres; higher-skilled operatives; improved design techniques and various architectural solutions.

Ductility verification is necessary to consider any long-term benefit from fibres. The design state-of-practice requires FRC to exhibit deflection softening behaviour, while deflection hardening is necessary to ensure that multi-cracking develops with controlled crack widths.

De Rivaz concluded that a prequalification test with a panel spray is required on the job site: “The energy absorption is based on EN 14488-5 plus the residual strength-based three-point bending test on a square panel with notch. This would be a global solution to ensure better quality control, safety and material properties.”

Sotiris Psomas, Director, Cowi UK

Sotiris Psomas gave an insight into SFRC for tunnel and shaft linings. He began with SFRC applications such as precast (PC) for high strength, primary or onepass linings; sprayed (SC) for moderate strength, primary and secondary linings; and cast-in-place (CIP) for all strengths, primary and secondary linings. Different tests exist in the UK, but BS EN 14561 has been mainly adopted for the derivation of design strength parameters of SFRC.

Psomas explained that fibre orientation and dispersion are affected by the mixing, casting procedure, formwork geometry dynamic characteristics (flow) and spraying direction.

The design – assisted by testing according to EN 1990 (cl 5.2) and fib MC2010 (cl 7.12) – can be adopted as a design approach by using full-scale testing (which can provide the real structural response); full-scale direct tensile strength, and crack spacing and crack-width development. In terms of sustainability, Psomas said that significant benefits from reductions in material use and waste, in addition to reduced construction time, are based on two large sewer projects in London (7.2m ID). The adoption of SFRC led to:

  • Savings of up to 2,000t of rebar per kilometre and led to a reduction in embodied energy of 28,000GJ/km;
  • A saving of 50% compared to the RC option, and
  • Embodied carbon of 1,700t/km CO2 – a saving of 35% compared to the RC option.

Further future savings can be achieved by adopting compatible ‘green’ concrete technologies such as polymer concrete.

“The design assisted by testing has served the UK tunnelling industry well in its adoption of SFRC,” said Psomas. “For statically indeterminate structures such as tunnel linings, it offers significant strain distribution capacity.

“For other structural elements in underground structures, where localised stress concentrations are present, a hybrid system comprising rebar and steel fibres represents an optimised solution.” Psomas added there is enough evidence suggesting that adequate durability for fibre-only reinforced structures can be achieved by SFRC.

He continued: “There is a need for standardisation on the design rules for SFRC (shaft and tunnel) linings and their incorporation into national standards. Structural verification for SFRC against shear needs to be further advanced; structural verification for hybrid against crack control needs to be furthered refined; and ‘green’ SFR concrete mixes are required to reduce environmental impact.”

Giovanna Cassani, Technical Director, Rocksoil

Adopting a historical approach Giovanni Cassani examined the oldest SFRC segmentally-lined metro tunnel in the world: Line 1 Lot 3 in Naples built between 1991 and 1992. The single-track running tunnel with a 6.40m OD was excavated using an openface shield with an articulated mantle by Markham & Co. The shield diameter was 6.55m, the rings had an average length of 1.2m and a diameter of 6.40m.

The project was constructed by main contractor Societa per la Progettazione e la Costruzione della Metropolitana di Napoli for the Municipality of Naples. Project management was by Metropolitana Milanese Strutture e Infrastrutture per il Territorio, while the sub-contractor was Metrosud Napoli.

Cassani’s slides showed that the geological formations comprised the typical yellow tuffs and loose Pozzuolanas. “At that time there were no standard references available to design SFRC,” Cassani said. “The only guidelines available were those from the Italian Association for Structural Concrete.”

Ultimate limit state (ULS) plus serviceability limit state (SLS) design represented a new, constitutive model for concrete, considering compression and traction strength. It was defined and adopted to calculate a new ULS resistance domain to apply to SFRC segments.

Different laboratory tests on SFRC were performed in order to define the design traction resistance of the concrete. During the precast segment design stage, AICAP guidelines (1982) for the design of SFRC were considered, as were material definition, cementitious matrix plus steel-fibre reinforcement, SFRC properties, concrete mix design, SFRC mixing and casting, design criteria, characterisation and control SFRC by test.

During the design stage, it was apparent that steel fibre can affect concrete behaviour in several ways, such as preventing and confining cracking, and potentially increasing concrete strength and ductility, which may be regarded as a structure’s ability to support substantial plastic strain with no loss of strength. In terms of ring assembly, jacking by the TBM generates bursting and spalling forces, while thrust loads acting on the tunnel cross-section induce bursting and spalling stresses in the segments’ radial joints.

Analysing the main benefits of steel fibres, they found a homogeneous distribution of steel fibres close to the surface with reinforcement at the joint; an increase of load bearing capacity to first crack at the joint, as well as good control of shrinkage cracks.

During the construction stage, the theoretical alignment of the tunnel was approximated by using proper sequences of two types of rings: type ASX-on the left hander; type BDX on the right hander with spacer and without rotation of the ring on its axis. The sequence of the rings was calculated and optimised using computer software. The difference between the theoretical line and the actual line of the tunnel could never be greater than 25mm. Excavating equipment was fitted with a special measurement system for tunnel advance which established the position of the machine with respect to the planned alignment. The data was used by the machine’s operator to minimise deviations from the planned axis.

Any overbreak around the rings is backfilled with grout injected directly through holes in the segments. The injection pressure is designed to make sure that the cavity between the ring and the ground is completely filled and grout does not find its way back inside the shield through the tail seal. Radial jointing using bolts ensures the transmission of normal and shear actions, but it cannot transmit moments except to a negligible extent. The succession of rings is considered a monolithic entity, and the moments calculated must be incremented on the basis of that hypothesis. The design moments are practically equal to double the calculated moments. For a tunnel section in tuff, the design of longitudinal and radial bolts was based on criteria relating to assembly operations and erection errors.

“As the oldest SFRC segmentally-lined metro tunnel in the world was designed and constructed almost 30 years ago without guidelines or specific codes for SFRC, the tunnel was used as a real-scale test for the technology,” said Cassani. “The overall geotechnical conditions were fairly good and the assumed behaviour of the SFRC was not so far from that expected for standard concrete. However, the tunnel is still operating and no repair or maintenance works were ever carried out.”

SFRC comes to France

SFRC was first used in large-scale precast segments in France on the Grand Paris Express project. Construction began in June 2016 and includes 180km of tunnels and 68 stations, all excavated by 24 TBMs.

Bernard Berge, key account manager of Bekaert Underground Solutions, explained: “The key to success was the partnership with actors [participants] involved in the project to guide and promote the use of FRC.”

Owner Societe du Grand Paris (SGP) conducted a feasibility study of SFRC precast segments for Line 16–Lot 1, which will be 19.3km long. Berge said that traditional reinforcement is very unlikely to be the cheapest and best solution because it is procured on the wrong basis.

“We see too often the client on one side, the contractor on the other side and the consultant in between,” Berge adds. “As a supplier, we have to encourage teamwork to promote the FRC along the whole process.”

The project’s technical teams comprised AFTES and ITA, as well as input from Eiffage, Salini Impregilo, Bouygues, Vinci, Implenia and Pizzarotti. Precast companies involved are Bonna Sabla, Capremib, Alliance and Stradal. The designers are Egis, Arcadis, Aecom and Systra, while the key influencer on SFRC testing is Rome University. Bonna Sabla did the precasting and the contractor is Eiffage.

Dave Hicks, Retired Consultant

Dave Hicks gave an insight into the manufacture of steel fibre reinforced precast tunnel segments. He outlined the types of steel fibres, including a selection of loose and collated fibres.

Hicks explained the difference between steel fibres for structural performance and polypropylene fibres for fire performance. Careful selection of the fibre type for the required concrete performance is crucial, as is dosing: steel fibres are typically dosed at 30kg/m3 concrete, whereas polypropylene micro fibres (15mm long and 33μm diameter) are typically dosed at 1 kg/m3.

Hicks also looked at the production of segments, citing Morgan Sindal’s Ridham precasting facility as an example.

The factory includes two carousel production lines each with a dedicated batching plant; 40 movable pallets on each production line; one or more moulds per pallet; nine work stations; 10-minute cycle time and a division of labour between work stations to achieve 10-minute moves.

Hicks explained: “To ensure highquality SFRC segment production, you need to have the right materials and mix design, as well as the right batching equipment, the high-quality moulds, well thought-out manufacturing processes, experienced operatives, robust quality assurance and quality control processes.”

John Corcoran, Tunnel Manager on Thames Tideway West

Discussing the operational aspects of installing SFRC final linings, John Corcoran referred to the Hammersmith Connection Tunnel, London, which is part of the Thames Tideway scheme. The tunnel will intercept sewer flows from Hammersmith pumping station, where on average two million cubic metres of untreated sewage is discharged into the River Thames each year.

The Hammersmith Connection tunnel includes sprayed-concrete primary linings and a crystallising agent in pre-blended mix to provide a water-repellent barrier. The secondary lining is fibre-reinforced.

The process comprised three phases: installation of the primary lining for initial loading; the primary and secondary lining acting compositely; the primary lining in compression, and the secondary lining in tension.

During the tunnel’s construction, the project team implemented some key innovations, such as the use of a fully hydrostatic programmable logistic controller (PLC)-driven formwork system to cast the secondary lining.

Summing Up

John Greenhalgh, sales manager at Bekaert, concluded the seminar with a brief summing up then bade everyone present farewell.